Digital document reproduction systems are well-known. These systems typically include a digital document generator that may be coupled to the reproduction system directly or through a computer network. Digital document generators include computers, scanners, or other devices that store or permit a user to define content for a digital document. The digital data are provided to a print engine so the controller of the engine may control the process. The reproduction system also includes a photoreceptor belt or drum that provides a rotating surface for the development and transfer of a latent image that corresponds to the digital document.
The latent image development begins with the charging of a portion of the photoreceptor belt at a charging station. The charged portion of the belt is advanced through an imaging/exposure station, where the data digital is provided as a signal to a raster output scanner. The raster output scanner selectively discharges the charged portion of the photoreceptor belt to form the latent image in correspondence with the document digital data. The photoreceptor belt then advances to a development station where toner is attracted to the latent image. More than one development station may be used for the development of color images so that different color toner materials may be applied to the latent image. Once the latent image is developed, the belt rotates to a transfer station where the toner on the latent image contacts a support sheet material, such as a sheet of paper. Typically, a corona generating device generates a charge on the backside of the support material so the toner particles are attracted to the support material and migrate from the latent image to the support material. A detack unit removes the support material from the photoreceptor belt and the belt moves through a cleaning station to remove the residual toner particles so that portion of the belt may be used for development of another latent image. The support sheet impregnated with toner particles moves to a fuser station where fuser and pressure rollers permanently fuse the toner particles to the support material. The support material sheet is then directed to a catch tray for the accumulation of support sheets bearing the images of the digital documents sent to the reproduction system.
To provide data for the control of this reproduction process, one or more densitometers or enhanced toner area coverage (ETAC) sensors may be provided after the development station(s) to measure the developed mass of toner applied to a unit area, sometimes called developed mass per unit area (DMA), on the photoreceptor belt or drum. The ETAC sensor includes one or more light emitting diodes (LEDs) for emitting light at a particular wavelength, which is preferably in the infrared range. The LEDs of the ETAC sensor are oriented at a particular angle with respect to the photoreceptor belt so that the emitted light is reflected by the toner on the photoreceptor belt and one or more photodetectors are located at the reflection angle to receive the light reflected from the photoreceptor belt. Typically, the latent image includes a toner control patch so the emitted light impinges on an area having toner to produce the toner density measurements. The voltage signal generated by a photodetector may be used to determine the DMA for the application of toner to the photoreceptor belt or drum.
The photodetectors are located in the area of reflected light so that one or more of the photodetectors receive specular light reflected from the photoreceptor. Other photodetectors are located so that they receive diffuse light reflected from the applied toner. The photodetectors generate a voltage signal that corresponds to the amount of light received by the photodetector. Thus, the photodetectors provide a specular measurement and a diffuse measurement. The specular measurement refers to light reflected by bare photoreceptor within the toner patch that presents a mirror surface to the emitted light, while the diffuse measurement refers to light reflected by the toner patch that is uneven and diffuses the emitted light from the LEDs. Both signals are important for reproduction control because the specular measurement is self-calibrating with LED intensity variations but saturates at typical solid area masses while the diffuse measurement remains sensitive to toner mass as it increases but is altered by LED intensity variations. Consequently, the specular signal has good signal to noise ratio characteristics for low DMA levels, while the diffuse signal has good signal to noise ratio characteristics for high DMA levels.
The controller of a digital reproduction system uses the specular and diffuse measurements received from the ETAC sensors to maintain image quality. In response to the detection of small amounts of toner dirt on the lens of a LED in an ETAC sensor or reflectance changes in the photoreceptor belt, the controller may increase the intensity of the LED in an ETAC sensor. However, the increase in LED intensity alters the diffuse signals and the DMA measurements obtained from an ETAC sensor. Because DMA measurements are critical for maintaining image quality, adjustments to the intensity of a LED in an ETAC sensor alter the DMA measurements derived from the ETAC sensor signals. Thus, the controller's regulation of DMA may become too inaccurate for acceptable image quality.